TIL that in Sparta, after the wedding, brides would have their hair cut short and be dressed in men's clothing so that they would appear less threatening to their groom during the wedding night.
So this claim seems well sourced, but I had to look it up because my bullshit detector always perks up when Sparta is discussed.
The surviving written sources are limited and largely from a non-Spartan viewpoint. Anton Powell wrote that to say the written sources are "'not without problems'... as an understatement would be hard to beat".
By Roman times, when a more organized and centralized polity was creating a pan-Mediterranean culture that had a lot to do with which works ended up preserved, Sparta was pretty much a theme park shell of its former self, and its former self is probably not so amazing anyway. In particular, their unusually thorough and unbalanced slave economy is overlooked when lionizing them, and their military record is rather underwhelming, possibly due to the incredible inefficiencies, even by ancient Greek standards, of said slave economy.
They really do get a lot more reverence today than they deserve. Like, I get most historical cultures don't stack up the best with modern moral standards applied to them, but like, based on what I understand about Sparta, if the place were somehow transplanted through history to the modern day as it's own country, it would probably end up some sanctioned pariah state out of sheer disgust for how it treats it's own citizenry and handles it's foreign diplomacy.
300 is entirely fantasy, the motivation of the story is that Sparta had a long important holiday that always landed on the same predictable days, and if anyone attacked their allies the attack was going to be at the start of that long holiday so that one army didn't show up, and they missed out on some very important battles, so at some point when this happened again, they sent out 300 men to do something for the war effort, and holding a bottle neck while a storm wiped out most of the threat was heavily publicized to counter the 'fucking Spartans and their several days long fucking holiday that isn't even on a lunar cycle so everyone fucking know exactly when it happens never Zesu bolted ass showing up with their little skirts and manly open mouth kissing army' kind of sentiment.